Photoreduction processing method of three-dimensional metal nanostructure

ABSTRACT

In a method of producing a metal structure by photoreducing metal ion, a substance capable of suppressing growth of metal crystal is added to a medium in which metal ion is dispersed to prevent growth of the metal crystal produced by photoreduction of the metal ion, thereby processing resolution of a metal structure formed of the metal crystal is improved

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a method of producing a metal structureby photoreduction of metal ion. More specifically, the present inventionrelates to a method of manufacturing a metal structure, wherebyprocessing resolution thereof is improved by suppressing growth of metalcrystal constituting the metal structure.

2. Background Art

In recent years, fine processing technologies using light such asoptical lithography technology and optical disk manufacturing technologyare widely utilized, and such technologies have been studied in avariety of fields.

For example, the fine processing technology using light, which is mostwidely applied at present, is the above-mentioned optical lithographytechnology. The optical lithography technology is a backbone technologyfor manufacturing a variety of electronic devices such as semiconductorchips. The technology relates to a massive copying technology using aphoto-transferring technology in principle, in which a metal in aspecified region is dissolved, separated out, or removed finally in achemical manner, thereby a desired metal pattern is formed as a metalstructure. Therefore, this method may be used only for two-dimensionalprocessing, and it is impossible to use the method to freely form ametal structure having a three-dimensional structure.

On the other hand, a technique for forming a metal pattern byirradiating directly a laser beam on a specified material is known as atechnology for forming a desired metal pattern as a metal structureother than the above-mentioned optical lithography technology. Morespecifically, there are a technique involving irradiating a focusedlaser beam onto a medium having metal nanoparticles dispersed therein,to thereby melt and bind the metal nanoparticles at the focal point ofthe laser beam, thereby a metal pattern is formed as a metal structure;and a technique involving irradiating focused light onto metal ion,thereby the metal ion is photoreduced and a metal body as separated out,thereby a metal pattern is formed as a metal structure.

Here, in the above-mentioned technique involving separating out themetal body by photoreducing the metal ion, an arbitrary metal patterncan be formed as a metal structure in response to the track of scannedfocused laser beam by scanning the focused laser beam which irradiatesthe material ion. Accordingly, a metal structure having athree-dimensional structure can be freely formed, and its applicablerange is extremely wide, thereby studies and developments have been madeupon the technique in a variety of fields in recent years.

A method of improving the processing resolution of the metal structurein the technique involving photoreducing the metal Ion to separate outthe metal body is also known.

In general, an absorption rate of light increases when metal structureis separated out, and hence there are many cases where the reactionproceeds explosively just at the moment when an increased a amount ofthe metal structure exceeds a certain threshold value. There is aproblem that, when such a phenomenon occurs, the photoreduction of themetal ions existing in the vicinity of the focal point proceeds atthe,same time, thereby the processing resolution of the metal structuredegrades. In the method disclosed in JP2006-31631A, a specified pigmentis added to a material, thereby an absorption spectrum and an absorptioncross section of a non-processed material are maintained at constant,and the processing resolution is prevented from degradation bypropagating energy of the laser beam to an area other than the focalpoint of the laser beam, besides photoreduction efficiency at the focalpoint of the laser beam is improved.

This method could improve the processing resolution to a micrometerorder, but a higher-precision nanometer-order processing resolution wasyet required.

SUMMARY OF INVENTION

An object of the present invention is to provide a method of producing ametal structure by photoreduction of metal ion, whereby processingresolution is significantly improved compared with conventionaltechniques. More specifically, an object of the present invention is toprovide a method of producing a metal structure, whereby processingresolution thereof is improved by suppressing growth of metal crystalconstituting the metal structure.

The inventors of the present invention have found that metal crystalproduced by photoreduction continues to grow for a while even afterlight irradiation is stopped, and grows into a several micrometer size,and that such a phenomenon restricts the processing resolution of themetal structure produced by photoreduction of metal ion. Then, theinventors have made extensive studies, and as a result, they have foundthat a substance which can suppress growth of metal crystal when thesubstance is contained in a medium in which metal ion is dispersed,thereby completed the present invention.

That is, the present invention is as follows.

[1] A method of producing a metal structure composed of metal crystal,comprising a step of irradiating a medium containing metal ion dispersedtherein with light, to thereby photoreduce the metal ion to producemetal crystal, wherein the medium contains a substance which blocksgrowth of the metal crystal.

[2] The method according to [1], wherein the substance has one or morefunctional groups selected from the group consisting of ionic functionalgroups and coordinating functional groups.

[3] The method according to [2], wherein the substance having the ionicfunctional group is represented by the general formula (I) or a saltthereof:

R—COOH   (I)

In the general formula (I), R¹ represents a saturated or unsaturatedhydrocarbon group in which any hydrogen atom may be replaced by one ormore substituents selected from the group consisting of carboxyl, amino,thiol, hydroxyl and cyano groups, and any —CH₂— may be replaced by—C(═O)— or —N(R²)—, and R² represents an alkyl group.

[4] The method according to [2], wherein the substance having the ionicfunctional group is represented by the general formula (II) or a saltthereof:

R²—NH₂   (II)

In the general formula (II), R¹ represents a saturated or unsaturatedhydrocarbon group in which any hydrogen atom may be replaced by one ormore substituents selected from the group consisting of carboxyl, amino,thiol, hydroxyl and cyano groups, and any —CH₂— may be replaced by—C(═O)— or —N(R²)—, and R² represents an alkyl group.

[5] The method according to [2], wherein the substance having thecoordinating functional group is represented by the general formula(III) or a salt thereof:

R¹—SH   (III)

In the general formula (III), R¹ represents a saturated or unsaturatedhydrocarbon group in which any hydrogen atom may be replaced by one ormore substituents selected from the group consisting of carboxyl, amino,thiol, hydroxyl and cyano groups, and any —CH₂— may be replaced by—C(═O)— or —N(R²)—, and R² represents an alkyl group,

[6] The method according to [2], wherein the substance having thecoordinating functional group is represented by the general formula (IV)or a salt thereof:

R¹—OH   (IV)

In the general formula (IV), R¹ represents a saturated or unsaturatedhydrocarbon group in which any hydrogen atom may be replaced by one ormore substituents selected from the group consisting of carboxyl, amino,thiol, hydroxyl and cyano groups, and any —CH₂— may be replaced by—C(═O)— or —N(R²)—, and R² represents an alkyl group.

[7] The method according to [2], wherein the substance having thecoordinating functional group is represented by the general formula (V)or a salt thereof:

R¹—CN   (V)

In the general formula (V), R¹ represents a saturated or unsaturatedhydrocarbon group in which any hydrogen atom may be replaced by one ormore substituents selected from the group consisting of carboxyl, amino,thiol, hydroxyl and cyano groups, and any —CH₂— may be replaced by—C(═O)— or —N(R²)—, and R² represents an alkyl group.

[8] The method according to [1], wherein the substance is represented bythe general formula (VI) or a salt thereof:

R¹—O—R³   (VI)

In the general formula (VI), R¹ and R³ each represents a saturated orunsaturated hydrocarbon group in which any hydrogen atom may be replacedby one or more substituents selected from the group consisting ofcarboxyl, amino, thiol hydroxyl and cyano groups, and any —CH₂— may bereplaced by —C(═O)— or —N(R²)—, and R² represents an alkyl group.

[9] The method according to [1], wherein the substance is represented bythe general formula (VII) or a salt thereof:

R¹—C(═O)—NH—R³   (VII)

In the general formula (VII), R¹ and R³ each represents a saturated orunsaturated hydrocarbon group in which any hydrogen atom may be replacedby one or more substituents selected from the group consisting ofcarboxyl, amino, thiol, hydroxyl and cyano groups, and any —CH₂— may bereplaced by —C(═O)— or —N(R²)—, and R² represents an alkyl group.

[10] The method according to [1], wherein the substance is a polymer ora copolymer composed of a monomer having one or more functional groupsselected from the group consisting of amino, carboxyl, carbonyl andthiol groups.

[12] The method according to any one of [1], wherein the metal ion is asilver ion.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing illustrating the action of a substance which blocksgrowth of a silver crystal.

FIG. 2 is an electron micrograph of a silver line obtained by addingNDSS as the substance which blocks growth of metal crystal to a medium.

FIG. 3 is an electron micrograph of a silver line obtained by adding nosubstance which blocks growth of metal crystal to a medium.

FIG. 4 is an electron micrograph of silver lines obtained by addingDL-alanine as the substance which blocks growth of metal crystal to amedium.

FIG. 5 is an electron micrograph of a silver line obtained by addingDL-alanine as the substance which blocks growth of metal crystal to amedium (enlarged micrograph of FIG. 4).

FIG. 6 is an electron micrograph of silver lines obtained by addingsodium decanoate as the substance which blocks growth of metal crystalto a medium.

FIG. 7 is an electron micrograph of silver lines obtained by addingdisodium sebacate as the substance which blocks growth of metal crystalto a medium.

FIG. 8 is an electron micrograph of silver lines obtained by addingsodium laurate as the substance which blocks growth of metal crystal toa medium.

FIG. 9 is an electron micrograph of silver lines obtained by addingDL-2-amino-n-octanoic acid as the substance which blocks growth of metalcrystal to a medium.

FIG. 10 is an electron micrograph of silver lines obtained by addingsodium N-lauroyl sarcosinate hydrate as the substance which blocksgrowth of metal crystal to a medium.

FIG. 11 is an electron micrograph of silver lines obtained by addingpoly(vinylpyrrolidone) as the substance which blocks growth of metalcrystal to a medium.

FIG. 12 is an electron micrograph of a silver rod formed by adding NDSSas the substance which blocks growth of metal crystal to a medium.

FIG. 13 is an electron micrograph of a silver rod formed by adding NDSSas the substance which blocks growth of metal crystal to a medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention are described indetail.

In the present invention, the “metal structure” refers to a structureformed of metal crystals produced by photoreduction of metal ion.Therefore, as the size of each metal crystal becomes smaller, theprocessing resolution of the metal structure may be improved. The shapeof the metal structure manufactured by the method of the presentinvention includes a one-dimensional structure such as a line or curve,a flat two-dimensional structure, and a spatial three-dimensionalstructure. According to the method of the present invention, a metalstructure can be formed in an arbitrary pattern at any part in a medium.For example, it is easy to produce the metal structure on a part orwhole of the surface of the medium or to form the metal structure at theinside thereof.

The light irradiated in the method of the present invention pro-idesenergy to reduce metal ion, therefore, the light to be irradiated has awavelength at which the metal ion has an absorption. However, in thecase where the medium contains a substance, which converts thewavelength of the irradiated light into the wavelength at which themetal ion has an absorption, the light to be irradiated is not limitedto light having the wavelength at which the metal ion has an absorption.Examples of the light source include laser light sources, light-emittingdiodes, and lamps, and the laser light sources are preferably usedbecause high energy is required to photoreduce the metal ion. Usually,the light to be irradiated is focused using a focus lens and focused ina medium in which metal ion is dispersed. Focusing of the light canincrease the photon density at the focal point to a very high level, andmay localize light energy necessary for reduction of the metal ion atthe focal point. As a result, the metal ion can be photoreduced only inthe vicinity of the focal point, and a fine metal structure can bemanufactured along the track of the focal point by scanning the focalpoint. Moreover, the light may be focused to perform fine processing ata size scale of the focal point. In addition, femtosecond short pulselaser beam, or the like may be used to provide higher light energy perunit time.

The intensity of the irradiated light required for photoreducing metalion depends on the type of the metal ion or the absorbance of the metalion at the wavelength of the irradiated light. That is, in the casewhere the absorbance of the metal ion at the wavelength of theirradiated light is low, the intensity of the irradiated light requiredfor photoreducing the metal ion is relatively high, while in the casewhere the absorbance of the metal ion at the wavelength of theirradiated light is high, the intensity of the irradiated light requiredfor photoreducing the metal ion is relatively low. Meanwhile, in thecase where the medium contains a substance which absorbs or scatters theirradiated light, the intensity of the irradiated light becomes smalleras the irradiated light passes through the medium. Therefore, it isnecessary to relatively increase the intensity of the irradiated light.Thus, the intensity of the irradiated light necessary for photoreducingthe metal ion may not be simply determined, but may be adjusted to arange of, for example, about 0.1 mW to 10 mW before incidence to themedium. It is not necessary to maintain the intensity of the irradiatedlight to a constant-level during processing, and the intensity mayappropriately be changed.

As well as the intensity of the irradiated light, the scan rate of thefocal point may appropriately be adjusted depending on the absorbance ofthe metal ion at the wavelength of the irradiated light or the presenceof a substance in the medium. Therefore, the scan rate necessary forphotoreducing the metal ion may not be simply determined, but may beadjusted to a range of, for example, about 0.1 μm/s to 100 μm/s. It isnot necessary to maintain the scan rate to a constant level duringprocessing, and the rate may appropriately be changed.

Scanning of the focal point may be performed by, for example,irradiating light to a medium, in which metal ion is dispersed, andwhich is placed on a XYZ-axis stage, while the stage is movedone-dimensionaly, two-dimensionally, and three-dimensionally. The focalpoint in the medium can be arbitrarily scanned by moving the stage.Also, the scanning of the focal point may also be performed byarbitrarily moving the position of the focal point in the medium, whilethe position of the medium, in which the metal ion is dispersed, isfixed. The medium in which the metal ion is dispersed and the focalpoint of the irradiated light can be moved simultaneously for scanningthe focal point.

To manufacture a metal structure having a three-dimensional structure byscanning the focal point, it is preferred that metal crystals producedin advance do not block the track of the focal point. To achieve this, ametal structure could be manufactured by the sequential reductions ofthe metal ions by continuously scanning the focal point from a part,which is far from the incidence position of the irradiated light to themedium, to a Dart near to the incidence position. However, if the trackof the focal point is not blocked, it is not necessary to scan the lightfrom the farthest part to the nearest part. The focal point may bescanned in such a manner to manufacture a fine metal structure having anarbitrary three-dimensional structure, and to easily manufacture even ahollow metal structure such as a box. Meanwhile, in the case where thetrack of the focal point may be blocked by the metal crystals producedin advance, there may be employed a method involving: vanishing theirradiated light; appropriately changing the incidence angle of theirradiated light to the medium; and starting irradiation from a positionnot to block the track of the focal point to scan the focal point again.

In the present invention, as the metal ions to be photoreduced, thefollowing are exemplified.

(1) Ions of transition elements from the IIIA group to IB group in theperiodic table. Of those, Cr ion, Mn ion, Fe ion, Co ion, Ni ion, Pdion, Pt ion, Cu ion, Ag ion, and Au ion are preferred.

(2) Ions of elements in the IIIB group. Of those, Al ion and In ion arepreferred.

(3) Zn ion, Cd ion, Hg ion, Na ion, K ion, Mg ion, and Ca ion.

The state in which metal ion to be photoreduced is dispersed in a mediumincludes, for example, a state in which metal ion is dissolved in anaqueous medium and a state in which metal ion is dispersed in a mediumsuch as an organic solvent or resin. The state in which metal ion isdispersed includes a state in which the ion is dispersed as a form ofcolloid or micelle.

In the present invention, the concentration of the metal ion dispersedin a medium is not particularly limited, and is preferably in the rangeof 0.001 M to 10 M. The concentration of the metal ion is morepreferably in the range of 0.01 to 1 M.

The medium which can be used in the present invention is notparticularly limited as long as metal ion can be dispersed therein, andthe medium includes: liquids or fluids such as water, organic solvents,and fats and oils; semisolids such as gel; and solids such as a resin(which is preferably a substance soluble in an organic solvent such asPMMA or FVA, or water), amorphous materials such as glass, inorganiccrystals which may be doped with metal ion, such as lithium niobate, andthe medium is preferably water. The medium in which metal ion isdispersed may be directly irradiated with light, or may be placed in acontainer or placed on a substrate and then irradiated with lightthrough the container or substrate. In the case where the medium inwhich metal ion is dispersed is a liquid or fluid, light may beconvergently irradiated on a contact surface between the container andthe medium or between the substrate and the medium to form a metalstructure on the inner surface of the container or on the substrate.

In the present invention, the “substance which blocks growth of metalcrystal” refers to a substance which prevents crystals formed byseparating out of a metal from binding together and blocking the metalcrystals from becoming larger. Examples of such a substance include asubstance having an effect of covering the surface of metal crystal toprevent binding of the metal crystal to another metal crystal (FIG. 1).Based on this perspective, the substance which blocks growth of metalcrystal preferably has all of the following properties.

(1) To have an atom which has affinity for or binds directly to a metalor metal ion in its molecule.

(2) To be dispersed in a solvent in which metal ion is dispersed.

(3) To have a low ability to directly reduce metal ion.

(4) To form no precipitates by binding to metal ion. However, if theprecipitates can be dissolved again by pH adjustment of the solvent orby formation of complex ion of metal ion, such a substance may be used.

Of those substances, which are considered to satisfy the above-mentionedconditions, the substance which can be used in the present invention isa hydrocarbon chain further having at least one of the followingproperties.

(5) To have an ionic functional group in its molecular structure.

(6) To have a coordination linkage functional group having an unsharedelectron pair (lone pair) in its molecular structure.

(7) To have a peptide bond or a similar structure thereof, an etherbond, or an ester bond in its molecular structure.

(8) To have a carbonyl group in its molecular structure.

The above-mentioned hydrocarbon chain includes a saturated orunsaturated hydrocarbon chain and preferably includes a saturatedhydrocarbon chain, because the saturated hydrocarbon has a lower abilityto reduce metal ion.

The chain length of the hydrocarbon is not particularly limited and mayappropriately be selected so that the hydrocarbon can be easilydispersed in a medium in consideration of the type of the medium to beused or the presence of a hydrophilic group in the molecular structure.In the case where water is used as a solvent and the molecular chainincludes no hydrophilic group, the length of the carbon chain ispreferably about 5 to 10.

The above-mentioned Ionic functional group refers to a functional groupwhich can be ionized in an aqueous solution and includes anionic andcationic functional groups. Examples of the anionic functional groupsinclude carboxyl, sulfonyl, phosphate, and silanol groups and saltsthereof, while examples of the cationic functional groups include aminoand pyridinium groups and salts thereof. Among the salts of the ionicfunctional groups, the salts of the anionic functional groups includesodium and potassium salts; while the salts of the cationic functionalgroups include halogenated salts.

Examples of the above-mentioned coordination linkage functional groupsinclude thiol, hydroxyl, and cyano groups. The thiol group can form athiol bond together with gold, silver, copper, or the like.

A substance having the above-mentioned properties has affinity for ametal or binds directly to a metal atom, and hence the substance cancoat the surface of the metal to prevent growth of metal crystal.

Substances represented by the following general formulae (I to VII) andsalts thereof are included in the substance which blocks growth of metalcrystal according to the present invention.

R¹—NH₂   (II)

R¹—SH   (III)

R¹—OH   (IV)

R¹—CN   (V)

R¹—O—R³   (VI)

R¹—C(═O)—NH—R³   (VII)

In the formulae, R¹ and R³ represent a saturated or unsaturatedhydrocarbon group in which any hydrogen atom may be replaced by one ormore substituents selected from the group consisting of carboxyl, amino,thiol, hydroxyl and cyano groups, and any —CH₂— may be replaced by—C(═O)— or —N(R²)—, and R² represents an alkyl group.

The chain length of the alkyl group represented by R² is notparticularly limited and may appropriately be selected in considerationof the type of a medium to be used and the presence of a hydrophilicgroup in the molecule structure. In the case where water is used as asolvent and the molecular chain includes no hydrophilic group, thelength of the carbon chain is preferably about 5 to 10.

Examples of the salts of the substances represented by the above generalformulae (I to VII) include substances having a sodium salt of acarboxyl group (—COONa), a potassium salt of a carboxyl group (—COOK), acalcium salt of a carboxyl group ((—COO)₂Ca), or a silver salt of acarboxyl group (—COOAg). In addition, the substances represented by theabove general formulae (I to VII) or salts thereof include substancesobtained by ionizing such substances, and the substance which blocksgrowth of metal crystal according to the present invention includes asubstance having, for example, —COO⁻ (produced by ionizing a carboxylgroup or a salt thereof), —NH₃ ⁺ (produced by ionizing an amino group),or —SO₃ ⁻ (produced by ionizing a sulfonic group).

Specific examples of the substance represented by the above generalformula (I) or the salts thereof include DL-alanine, decanoic acid,sodium decanoate, sebacic acid, disodium sebacate, lauric acid, sodiumlaurate, DL-2-amino-n-octanoic acid, sodium

DL2-amino-n-octanoate, N-decanoyl sarcosic acid, sodium N-decanoylsarcosinate, N-lauroyl sarcosic acid, and sodium-lauroyl sarcosinate.

Specific examples of the substance represented by the above generalformula (II) and the salts thereof include amines such as 1-butyl amineand 1-hexyl amine.

Specific examples of the substance represented by the above generalformula (III) and the salts thereof include thiols such as 1-butanethiol and 2-aminoethane thiol.

Specific examples of the substance represented by the above generalformula (IV) and the salts thereof include some alcohols such as6-amino-l-propanol and butanol.

Specific examples of the substance represented by the above generalformula (V) and the salts thereof include butyronitorile.

Specific examples of the substance represented by the above generalformula (VI) and the salts thereof include a substance having an etherbond such as 3,3′-oxydipropionitorile.

Specific examples of the substance represented by the above generalformula (VII) and the salts thereof include peptides such as a diner ofalanine.

In addition, a polymer or copolymer formed of a monomer having one ormore functional groups selected from amino, carboxyl, carbonyl, thiol,hydroxyl, and cyano groups is also included in the substance whichblocks growth of metal crystal according to the present invention.Specific examples of the substance include poly(vinylpyrrolidone). Themolecular weight of the polymer or copolymer is preferably about 40,000to 80,000.

The concentration of the substance which blocks growth of metal. crystalin a medium is not particularly limited and is preferably in the rangeof 0.001 M to 10 M. The concentration is more preferably in the range of0.01 M to 1 M.

The temperature at which a metal structure is produced in the presentinvention is not particularly limited but is preferably in a temperaturerange in which the medium can maintain its original properties. Forexample, in the case where water is used as a medium, water freezes at atemperature below zero and evaporates at too high temperature.Therefore, it is impossible to maintain the original properties ofwater, and metal ion is reduced only at a high temperature, which is notpreferable. In such case, the reaction temperature is preferably about 5to 60° C., and usually, processing can be performed at room temperature.If optical energy is absorbed by metal ion, or the like, the energy isconverted into heat, which may cause an increase in the temperature ofthe medium during processing. Therefore, if necessary, a coolingapparatus may be used to suppress an increase in the temperature.

EXAMPLES

Hereinafter, the present invention will be explained in more detail inexamples described below. However, the scope of the present invention isnot limited to the examples.

Example 1

Shape of Metal Structure Obtained by Adding a Substance which BlocksGrowth of Metal Crystal to the Medium

Sodium N-decanoyl sarcosinate (NDSS) (formula VIII) was added to anaqueous solution of silver nitrate, and the solution was dropped on aglass substrate (Micro Cover Glass, manufactured by Matsunami GlassInd., Ltd.). Laser beam (light source: Titanium: sapphire femtosecondlaser (Tsunami (registered trademark), manufactured by Spectra-PhysicsK.K.), center wavelength: 800 nm, pulse width: 80 fsec), which wascontrolled to be focused on the upper surface of the glass substrate,was irradiated from the bottom of the glass substrate and the focalpoint was scanned linearly in a horizontal direction to the surface ofthe substrate. The final concentration of NDSS was 0.1 M, theconcentration of silver nitrate was 0.05M, the strength of theirradiated laser beam was 0. 8 mW, and the scan rate was 7 μm/s.Processing was performed at room temperature.

CH₃(CH₂)₈—CO—N(CH₃)—CH₂—COONa   (VIII)

NDSS Comparative Example Shape of Metal Structure Obtained by Adding NoSubstance Which Blocks Growth of Metal Crystal to the Medium

A solution obtained by adding a solution of Coumarin 400 (manufacturedby Exciton, purchased from Tokyo Instruments, Inc.) in 0.01 wt % ethanolto an aqueous solution of silver nitrate was dropped on a glasssubstrate (Micro Cover Glass, manufactured by Matsunami Glass Ind.,Ltd.). Laser beam (light source: Titanium: sapphire femtosecond laser(Tsunami (registered trademark), manufactured by Spectra-Physics K.K.),center wavelength: 800 nm, pulse width: 80 fsec), which was controlledto be focused on the upper surface of the glass substrate, wasirradiated from the bottom of the glass substrate, and the focal pointwas scanned linearly in a horizontal direction to the surface of thesubstrate. The concentration of silver nitrate was 0.05 M, the strengthof the irradiated laser beam was 0.8 mW, and the scan rate was 7 μm/s.Processing was performed at room temperature.

FIG. 2 shows an electron micrograph of a linear silver structure (silverline) formed of silver crystals formed on the substrate by the method ofExample 1, and FIG. 3 shows an electron micrograph of a silver lineformed on the substrate by the method of Comparative Example. FIG. 2indicates that the formed silver line has a width of about 150 nm andthat the particle of each silver crystal in the silver line has ananometer size. On the other hand, FIG. 3 indicates that the silver linehas a width of about 1 μm and that the surface is bumpy because of largerocky aggregates formed by growth of small particulate silver crystals.If such large aggregates are present, it is difficult to form a linewith a smaller width by the method of Comparative Example, because thefinal line width depends on the sizes of the large particles.

Further it should be noted that, although the size of the laser focalpoint is about 1 μm, a silver line with a width about ten times smallercould be drawn by the method described in Example 1.

FIG. 2 indicates that the particle size of each silver crystal is verysmall compared with the width of the silver liner which suggests that itis possible to achieve finer processing if the spot size of the focalpoint of the laser beam is controlled to be smaller or if light isirradiated only to a smaller region.

Examples 2 to 8

The same experiments were performed using aqueous solution of silvernitrate, to which various substances according to the present invention(the following chemical formulae IX to XV) were added instead of NDSSused in Example 1, to thereby form silver lines- The silver lines thusformed are shown in FIGS. 4 to 11. Experimental conditions of respectiveexamples are shown in Table 1. Processing was performed at roomtemperature.

Example 2

NH₂—CH(CH₃)—COOH   (IX)

DL-Alanine Example 3

CH₃(CH₂)₈—COONa   (X)

Sodium Decanoate Example 4

NaCOC—(CH₂)₈—COONa   (XI)

Disodium Sebacate Example 5

CH₃(CH₂)—₁₀—COH   (XII)

Sodium Laurate Example 6

CH₃—CH₂)₅—CH(NH₂)—COOH   (XIII)

DL-2-amino-n-octanoic Acid

Example 7

CH₃—(CH₂)₁₀—CO—N(CH₃)—CH₂—COONa.xH₂O   (XIV)

Sodium N-lauroyl Sarcosinate Hydrate Example 8

Poly(vinylpyrrolidone)

TABLE 1 Final concentration of substance which blocks growth of metalcrystal, concentration of silver nitrate, laser intensity at focalpoint, and scan rate Concentration Intensity of of substanceConcentration of irradiated Example in medium silver nitrate laser beamScan rate 2  0.1M 0.05M  2.0 mW  6 μm/s 3 0.09M 0.3M 1.2 mW 10 μm/s 40.09M 0.3M 1.2 mW 10 μm/s 5 0.09M 0.3M 1.2 mW 10 μm/s 6 0.09M 0.3M 0.8mW 10 μm/s 7 0.09M 0.3M 1.2 mW 10 μm/s 8 2 wt % 0.05M  1.6 mW  6 μm/s

In all the examples, the widths of the silver lines were in the range of200 to 300 nm, which are very small compared with the size in the caseof the comparative example. Meanwhile, also in the case where sodiumsorbate was added as an unsaturated hydrocarbon, a line was formed.However, the metal ion was reduced directly by the sodium sorbate withtime, which suggests that, in the case where a substance containing anrun saturated hydrocarbon is used, it was necessary to offset thereduction ability of the unsaturated hydrocarbon by, for example, addingan antioxidant immediately after processing, or previously adding anantioxidant to a material containing the metal ion.

Example 9

Formation of Three-Dimensional Structure

A metal structure having a three-dimensional structure was formed by themethod of producing a metal structure of the present invention.

NDSS (Chemical formula V) was added to an aqueous solution of silvernitrate, and the solution was dropped on a glass substrate (Micro CoverGlass, manufactured by Matsunami Glass Ind. Ltd.). Laser beam (lightsource: Titanium:sapphire laser (Tsunami (registered trademark),manufactured by Spectra-Physics K.K.), center wavelength: 800 nm, pulsewidth: 80 fsec), which was controlled to be focused on the upper surfaceof the glass substrate (spot size of focal point is about 1 μm), wasirradiated through the glass substrate from the bottom of the glasssubstrate, and the focal point was scanned linearly in a perpendiculardirection to the surface of the substrate. The final concentration ofNDSS was 0.1 M, the concentration of silver nitrate was 0.04 M, thestrength of the irradiated laser beam was 1.21 mW, and the scan rate was2 μm/s (first round) and 4 μm/s (second round).

FIG. 12 shows an electron micrograph taken from obliquely above of acylindrical silver structure (silver rod) which is formed of silvercrystals formed on a substrate under the condition at the first roundand stands upright on the substrate. This confirms that a silver rodhaving a cross section diameter of about 300 nm and having a smoothsurface was formed. The magnified micrograph of a silver rod obtained byprocessing under the conditions at the second round (FIG. 13) shows thata silver rod with the finest width of about 100 nm was successfullyprocessed.

The result demonstrated that, according to the present invention, afiner metal structure having a three-dimensional structure can be easilyformed only by controlling the direction of light scanning.

INDUSTRIAL APPLICABILITY

According to the present invention, the particle size of metal crystalproduced by photoreduction of metal ion can be controlled to a nanometersize, thereby the method significantly improves the processingresolution of a metal structure formed of the metal crystal. Therefore,an arbitrary pattern of fine and precise three-dimensional structure ofa metal structure can be formed easily. The present invention can beused for manufacture of a micromachine, formation of a magnetic field ina microspace by forming of a small metal coil, or control of therefractive index.

While the invention has been described in detail with reference topreferred embodiments thereof, it will be apparent to one skilled in theart that various changes can be made, and equivalents employed, withoutdeparting from the scope of the invention. Each of the aforementioneddocuments including the priority application JP2008-077913 isincorporated by reference herein in its entirety.

1. A method of producing a metal structure composed of metal crystal,comprising a step of irradiating a medium containing metal ion dispersedtherein with light, to thereby photoreduce the metal ion to producemetal crystal, wherein the medium contains a substance which blocksgrowth of the metal crystal.
 2. The method according to claim 1, whereinthe substance has one or more functional groups selected from the groupconsisting of ionic functional groups and coordinating functionalgroups.
 3. The method according to claim 2, whereon the substance havingthe ionic functional group is represented by the general formula (I) ora salt thereof:R¹—COOH   (I) In the general formula (I), R¹ represents a saturated orunsaturated hydrocarbon group in which any hydrogen atom may be replacedby one or more substituents selected from the group consisting ofcarboxy, amino, thiol, hydroxyl and cyano groups, and any —CH₂— may bereplaced by —C(═O)— or —N(R²)—, and R² represents an alkyl group.
 4. Themethod according to claim 2, wherein the substance having the ionicfunctional group is represented by the general formula (II) or a saltthereof:R¹—NH₂   (II) In the general formula (II), R¹ represents a saturated orunsaturated hydrocarbon group in which any hydrogen atom may be replacedby one or more substituents selected from the group consisting ofcarboxyl, amino, thiol, hydroxyl and cyano groups, and any —CH₂— may bereplaced by —C(═O)— or —N(R²)—, and R² represents an alkyl group.
 5. Themethod according to claim 2, wherein the substance having thecoordinating functional group is represented by the general formula(III) or a salt thereof:R¹—SH   (III) In the general formula (III), R¹ represents a saturated orunsaturated hydrocarbon group in which any hydrogen atom may be replacedby one or more substituents selected from the group consisting ofcarboxyl, amino, thiol, hydroxyl and cyano groups, and any —CH₂— may bereplaced by —C(═O)— or —N(R²)— and R² represents an alkyl group.
 6. Themethod according to claim 2, wherein the substance having thecoordinating functional group is represented by the general formula (IV)or a salt thereof:R¹—OH   (IV) In the general formula (IV), R¹ represents a saturated orunsaturated hydrocarbon group in which any hydrogen atom may be replacedby one or more substituents selected from the group consisting ofcarboxyl, amino, thiol, hydroxyl and cyano groups, and any —CH₂— may bereplaced by —C(═O)— or —N(R²)—, and R² represents an alkyl group.
 7. Themethod according to claim 2, wherein the substance having thecoordinating functional group is represented by the general formula (V)or a salt thereof:R¹—CN   (V) In the general formula (V), R¹ represents a saturated orunsaturated hydrocarbon group in which any hydrogen atom may be replacedby one or more substituents selected from the group consisting ofcarboxyl, amino, thiol, hydroxyl and cyano groups, and any —CH₂— may bereplaced by —C(═O)— or —N(R²)—, and R² represents an alkyl group.
 8. Themethod according to claim 1, wherein the substance is represented by thegeneral formula (VI) or a salt thereof:R¹—O—R³   (VI) In the general formula (VI), R¹ and R³ each represents asaturated or unsaturated hydrocarbon group in which any hydrogen atommaybe replaced by one or more substituents selected from the groupconsisting of carboxyl, amino, thiol hydroxyl and cyano groups, and any—CH₂— may be replaced by —C(═O)— or —N(R²)—, and R² represents an alkylgroup.
 9. The method according to claim 1, wherein the substance isrepresented by the general formula (VII) or a salt thereof:R¹—C(═O)—NH—R³   (VII) In the general formula (VII), R¹ and R³ eachrepresents a saturated or unsaturated hydrocarbon group in which anyhydrogen atom may be replaced by one or more substituents selected fromthe group consisting of carboxyl, amino, thiol, hydroxyl- and cyanogroups, and any —CH₂— may be replaced by —C(═O)— or —N(R²)—, and R²represents an alkyl group.
 10. The method according to claim 1, whereinthe substance is a polymer or a copolymer composed of a monomer havingone or more functional groups selected from the group consisting ofamino, carboxyl, carbonyl and thiol groups.
 11. The method according toany one of claim 1, wherein the metal ion is a silver ion.